Both mesoporous inorganic material and micropborous inorganic material are characterized by a large specific surface area in pores and are used in a large number of applications of considerable commercial importance. In most of these applications, the fact that the phase interface between the solid porous materials and the medium (liquid or gas) in which it is used is large can be very important. For example, these porous inorganic materials are often used as catalysts and catalyst supports in hydrocarbon conversion processes. Also, these porous inorganic materials are often used as adsorbents for the selective adsorption in the gas or liquid phase or the selective separation of ionic compounds. As used herein, the terms “porous inorganic materials” and “porous materials” includes solid mesoporous inorganic material, solid microporous inorganic material, and mixtures thereof.
Although a large phase interface is often a fundamental requirement for use of porous materials in different applications, a number of additional requirements related to the particular area of application are imposed on these materials. For example, the large phase interface available in the pores of the porous inorganic material must be accessible and useable. In many applications, size and shape of the macrostructures containing the porous inorganic material and the degree of variation of these properties can be decisive importance. During use, the size and shape of the macrostructures can influence properties like mass transport within the structures, pressure drop over a bed of particles of the material, and the mechanical and thermal strength of the material. Techniques that permit production of a material with increased specific surface area, pore structure (pore size/pore size distribution), chemical composition, mechanical and thermal strength, as well as increased and uniform size and shape, are consequently required to tailor porous inorganic macrostructures to different applications.
Mesoporous inorganic materials include amorphous metal oxide (non-crystalline) materials which have mesoporous and optionally partially microporous structure. The pore size of the mesoporous inorganic material is usually in the range of from about 20 Å to about 500 Å.
Microporous inorganic materials include crystalline molecular sieves. The pore size of crystalline microporous molecular sieves is usually in the range of from about 2 Å to about 20 Å. Crystalline microporous molecular sieves, both natural and synthetic, such as zeolites, have been demonstrated to have catalytic properties for various types of hydrocarbon conversion processes. In addition, the crystalline microporous molecular sieves have been used as adsorbents and catalyst carriers for various types of hydrocarbon conversion processes, and other applications. These molecular sieves are ordered, porous, crystalline material having a definite crystalline structure as determined by x-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. The dimensions of these channels or pores are such as to allow adsorption of molecules with certain dimensions while rejecting those with larger dimensions. The interstitial spaces or channels formed by the crystalline network enable molecular sieves to be used as molecular sieves in separation processes and catalysts and catalyst supports in a wide variety of hydrocarbon conversion processes.
Molecular sieves can be classified into various groups by their chemical composition and their structure. One group of molecular sieves is commonly referred to as zeolites. Zeolites are comprised of a lattice of silica and optionally alumina combined with exchangeable cations such as alkali or alkaline earth metal ions. Although the term “zeolites” includes materials containing silica and optionally alumina, the silica and alumina portions may be replaced in whole or in part with other oxides. For example, germanium oxide, titanium oxide, tin oxide, phosphorous oxide, and mixtures thereof can replace the silica portion. Boron oxide, iron oxide, titanium oxide, gallium oxide, indium oxide, and mixtures thereof can replace the alumina portion. Accordingly, the terms “zeolite”, “zeolites” and “zeolite material”, as used herein, shall mean crystalline microporous molecular sieves including molecular sieves containing silicon and, optionally, aluminum atoms in the crystalline lattice structure thereof, molecular sieves which contain suitable replacement atoms for such silicon and aluminum and ALPO-based molecular sieves which contain framework tetrahedral units of alumina (AlO2) and phosphorous oxide (PO2) and, optionally, silica (SiO2). Examples of ALPO-based molecular sieves include SAPO, ALPO, MeAPO, MeAPSO, ELAPO, and ELAPSO. The term “aluminosilicate zeolite”, as used herein, shall mean zeolites consisting essentially of silicon and aluminum atoms in the crystalline lattice structure thereof.
Many times it is desirable to carry out hydrocarbon conversion processes using small particles, e.g., crystals. The term “small crystals”, is used herein to mean the crystals have a diameter less than about 1 micron. For example, small crystal zeolites can have advantages over larger crystals when used as a catalyst, or catalyst base for reactions involving hydrocarbon conversion, because of their enhanced ratio of surface area to mass, high diffusion rates and reactivities, and resistance to deactivation by pore plugging and surface contamination. For similar reasons, they can have advantages in hydrocarbon separations.
Prior to using the porous inorganic material, especially crystalline microporous molecular sieves such as zeolites, in hydrocarbon conversion, the material is usually formed into structures, e.g., aggregates, such as pills, spheres, tablets, pellets, or extrudates. For example, although zeolite crystals have good adsorptive properties, their practical applications are very limited because it is difficult to operate fixed beds with zeolite powder. Therefore, prior to using the zeolite crystals in commercial processes, mechanical strength is conventionally conferred on the zeolite crystals by forming a zeolite aggregate such as a pill, sphere, or extrudate which usually is a dimension greater than 0.01 mm. The extrudate can be formed by extruding the zeolite crystals in the presence of a non-zeolitic binder and drying and calcining the resulting extrudate. The binder materials used are resistant to the temperatures and other conditions, e.g., mechanical attrition, which occur in various hydrocarbon conversion processes. It is generally necessary that the zeolite be resistant to mechanical attrition, that is, the formation of fines which are small particles, e.g., particles having a size of less than 20 microns. Examples of suitable binders include amorphous materials such as alumina, silica, titania, and various types of clays. Aggregates can also be formed without amorphous binder by compressing the crystals together in such a way that they become physically self bound.
Although such bound zeolite aggregates have much better mechanical strength than the zeolite powder, when the bound zeolite is used in a catalytic conversion process, the performance of the catalyst, e.g., activity, selectivity, activity maintenance, or combinations thereof, can be reduced because of the amorphous binder. For instance, since the binder is typically present in amounts of up to about 60 wt. % of the bound catalyst, the amorphous binder dilutes the adsorptive properties of the aggregate. In addition, since the bound zeolite is prepared by extruding or otherwise forming the zeolite with the amorphous binder and subsequently drying and calcining the extrudate, the amorphous binder can penetrate the pores of the zeolite or otherwise block access to the pores of the zeolite, or slow the rate of mass transfer to and from the pores of the zeolite which can reduce the effectiveness of the zeolite when used in hydrocarbon conversion processes and other applications. Furthermore, when a bound zeolite is used in catalytic conversion processes, the amorphous binder may affect the chemical reactions that are taking place within the zeolite and also may itself catalyze undesirable reactions which can result in the formation of undesirable products.